mxs: - update uboot-mxs to 3.10
 - enable building uboot for imx23 along sbImage by default
 - (doc updated)

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>



git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38781 3c298f89-4303-0410-b956-a3cf2f4a3e73